More than 700 people have died from the swine flu virus worldwide since H1N1 emerged in April, the World Health Organisation (WHO) said.

Twenty-nine of those deaths have been in the UK but the WHO is no longer giving country-by-country breakdowns.

The global death toll is about 300 up on the 429 reported two weeks ago but since then countries have been told there is no longer any need to report infections.

The figure compares with 262 confirmed deaths from bird flu in 15 countries since 2003.

The WHO report of deaths came as Britain's chief medical officer, Sir Liam Donaldson, said pregnant women, cancer patients on chemotherapy and others with weakened immune systems may be advised to stay away from crowds for "a few weeks" when the swine flu pandemic reaches its height, probably this autumn.

Mothers-to-be are at present not being recommended to cut back on normal activities such as going to work, using public transport or attending events and family gatherings, but that could change if swine flu reached a level of, for example, one in three of the population.

But Donaldson also said some pregnant women may wish to exercise their choice now "on a highly precautionary basis, to avoid large, densely populated gatherings where they have little control over personal contact".

Donaldson has been attempting to clarify official advice since confusion emerged at the weekend over exactly what it meant.

"We are not advising pregnant women to cut down on their normal daily activity – some might choose to be very precautionary and not want to go into crowded places, but that is not the advice," he said.

"But we will look at it again when it comes to the autumn when we get possibly big numbers of cases.

"If we got, for example, one in three of the population affected by flu, which is one of the estimates, at that point I may advise pregnant women and people for example on cancer treatment who have weakened immune systems to avoid crowded places for a period of a few weeks when it is at its peak."

His remarks came as ministers were urged to rethink their policy of keeping schools open through the pandemic after research showed that a shutdown would curb the spread of infection and limit the number of deaths.

Two infectious disease experts said school closures should be considered to reduce the number of cases and buy time until a vaccine is available.

Schools across Britain have broken up for summer holidays and experts hope this will help to slow the spread of the virus. But there are fears that when classes resume in the autumn the number of cases will increase rapidly.

School closures would cause serious difficulties for working parents, lead to a 1% loss in GDP through absenteeism and see as many as 30% of NHS staff taking time off just when they are needed to treat patients.

In a study published in the Lancet Infectious Diseases, government adviser Prof Neil Ferguson and Dr Simon Cauchemez, both of the department of infectious disease epidemiology, Imperial College London, said "prolonged" closures could reduce the scale of the outbreak by 13-17% and at the pandemic's peak the shutting of schools could bring down the number of cases by 38-45%.

"It is therefore hoped that closure of schools during the pandemic might break the chains of transmission, with the following potential benefits: reducing the total number of cases; slowing the epidemic to give more time for vaccine production; and reducing the incidence of cases at the peak of the epidemic, limiting both the stress on healthcare systems and peak absenteeism in the general population, and thus increasing community-wide resilience," the researchers said.

Such a move would also raise the question of what should be done with millions of schoolchildren during a prolonged shutdown, they added. The authors said that governments in Europe and America might have to take such a step after they studied the impact of school closures during flu epidemics in other countries stretching back to 1918.

They say that study of the 1918 flu outbreak in America and Australia indicates that shutting schools, in tandem with closing churches and improved hygiene, could have reduced the death toll by between 10% and 30%, and as much as 50% in some cities at the height of the outbreak.

About 100 schools closed after the start of the outbreak in May but soon reopened when official advice changed because swine flu was becoming so prevalent.

Donaldson responded coolly to the idea during his GMTV appearance. "I think it would take a lot for us to move in that direction. It would be extremely disruptive to society. When would you open them again, given that flu might be around for several months?

"If we look at what we did in the west Midlands for example, where we did very aggressively initially close schools and treat people with Tamiflu who didn't have symptoms but were contacts of cases, eventually it broke out of the box and spread more widely."

He added: "I think we will obviously keep all of these things under review as we do with any scientific advice, but at the moment I think it is unlikely."

The WHO said "it is really up to individual countries to consider what mitigation measures suit them in regard to the situation in individual countries".